1. Field of the Invention
This invention relates to a photomask with a phase shifter which uses a photolithographic phase shift technique to manufacture semiconductors, and a method of fabricating semiconductor devices by using such a photomask.
2. Description of the Related Art
With semiconductor device fabrication, remarkable able progress has been made in manufacture more minute components. In photolithography, an optical stepper is improved to increase the numerical aperture, and an exposing source is also improved to use shorter wavelengths. Refined and sophisticated resists also contribute to make more minute semiconductor devices. A photo-processing technique established for the manufacture of semiconductor devices uses a half-micron process rule, to take advantage of the improvement of the foregoing techniques.
The more the numerical aperture is increased to cope with the foregoing trend, the less the focus tolerance of the optical stepper. Minimization of the components fabricated by a light exposure process is reaching its limit so that it will become more difficult to prepare resist patterns necessary for manufacturing semiconductor devices. This inconvenience is caused by the diffraction effect of light.
FIG. 11(A) of the accompanying drawings shows a cross-section of a photomask, also called a reticle, used with an optical stepper. The photomask comprises quartz glass 80, having a plurality of chromium patterns 82 for shielding light, and light passing regions 84 interposed between the chromium patterns 82. When the light passing regions 84 are densely packed for minimization of circuit patterns, rays of light on a wafer cannot be separated due to the diffraction effect of light. Specifically, light has the phase as shown in FIG. 11(B). Light intensity is equal to the square of the amplitude of the lightwave. When the adjacent patterns are densely packed, light intensity tends to appear at the shielded portion due to the diffraction effect, as shown in FIG. 11(C).
A phase shift technique based on another feature of light has been developed recently as a novel method to further minimize semiconductor devices. FIG. 12(A) shows a cross-section of a photomask made by applying the phase shift technique. This photomask is characterized by a shifter 94 which is located at one of light passing regions 96a, 96b to shift the phase of light. The photomask made of quartz glass 90 has a plurality of chromium patterns 92. A resist compound is applied over the chromium patterns 92 so that the chromium patterns 92 are transferred to the resist by exposing an electron beam or laser beam and by development and etching processes.
As shown in FIG. 12(B), the light passing through the photomask has a phase on a wafer which is reversed 180 degrees by the shifter 94 at the light passing region 96a. There is light having intensity due to the diffraction effect at a region shielded by the chromium pattern 92. However, since the intensity is equal to the square of amplitude of, the light intensity on the wafer is always zero between the adjacent light passing regions as shown in FIG. 12(C). The exposed and developed resist patterns are completely separated since the photomask made by applying the phase shift technique can provide very high resolution without difficulty.
One of conventional methods to make photomasks by the phase shift technique is shown in FIGS. 13(A) to 13(E). FIG. 13(A) is a cross-sectional view of a photomask having chromium patterns 92 on quartz glass 90. SOG98 usually coated on the quartz glass 90 as material to make a shifter 94. Thickness d of SOG98 to shift the phase of light by 180 degrees is given by EQU d=.lambda./[2.times.(n.sub.1 -n.sub.2)]
where .lambda. is a wavelength, n.sub.1 is a refractive index of a shifter, and n.sub.2 is a refractive index of an ambience around the photomask.
The refractive index n.sub.2 is approximately 1 (one) when the photomask is placed in air. When g-ray of a mercury lamp is used as exposing light, the refractive index of SOG is 1.45. Since the g-ray has a wavelength of 0.436 .mu.m, the thickness of the shifter 94 is 0.484 .mu.m from the above equation.
A resist 99 is coated on SOG98 the thickness d as shown in FIG. 13(C). Then an electron is applied to the resist 99 to draw and expose the chromium patterns 92, being developed as shown in FIG. 3(D). SOG98 undergoes dry etching through the resist 99 as a mask (FIG. 13(E)). Unnecessary resist 99 is stripped by sulphuric acid. Thus, a shifter 94 is obtained as shown in FIG. 12(A).
The foregoing conventional method has following drawbacks. SOG98 must be coated after forming the chromium patterns 92 on the quartz glass 90. The resist 99 must then coated on SOG98. After this, the printing, exposure, development and etching processes must be carried out. This means there are a number of steps to be taken to fabricate photomasks.
Coating of SOG98 not only increases the fabrication steps but also adversely affects determination of the precise thickness of the shifter 94. This is because SOG98 is usually applied over the quartz glass 90 and the chromium patterns 92 by spincoating. SOG98 cannot be uniformly coated on them since the chromium patterns 92 project from the quartz glass 90. The phase shift function of the shifter 94 depends upon the thickness of SOG98. Therefore, the shifter 94 of the conventional method suffers from varying phase shift because of non-uniform thickness of SOG98. The phase shift should be usually controlled in the range of 180.degree..+-.10.degree.. The conventional shifter 94 made by applying SOG98 satisfy this requirement.